Good question. How much memory on RPi3 and how much required to run Windows10?

RPi3 has 1 GiB of RAM. The system requirements of Windows 10 (cf. https://www.microsoft.com/en-us/windows/windows-10-specifica...) are 1 GiB for 32 bit and 2 GiB for 64 bit (both on x86). So I think if Microsoft is willing to, it should work, in particular if they slim down the fictional RPi3 version of Windows 10 (which I would strongly recommend).

I personally see the much larger performance regression for Windows 10 on RPi3 in the fact that at least on Windows 10 IoT Core there was no hardware acceleration of graphics on RPi3 when I last tested it (about a year ago), which caused it to feel rather sluggish.

I guess not. It requires hardware features from Snapdragon 820/835.

Do you know particular details what these hardware features are?

I'm guessing it's both a performance issue and a "let's not support too many different ARM chips from the beginning" issue.

Snapdragon 835 will probably be the minimum level of ARM performance Microsoft will accept. We may start seeing Samsung's Exynos and a high-end MediaTek processor being supported in a year or two.